You'll have to make your way through a maze of trees and stalactites. Some of the groups of cult members will flee when you approach, and some will try to stop you. I think it depends on what your reputation is.
Make your way from the Arrival Platform down to the south edge of the map and the Subterranean docks. Once you've cleared the docks take one of the boats and make your way over the water to the eastern side of the Cult of the Sacred Items. There are two rows of rocks leading towards the small island, go between them and enter the dock area of the cult, since this is the easiest way to enter.
You'll be teleporting out of the Cult's base, so dont' worry that you can't see any exits from this area.